SmCo-BASED ALLOY NANOPARTICLES AND PROCESS FOR THEIR PRODUCTION

ABSTRACT

SmCo-based alloy nanoparticles composed mainly of a SmCo-based alloy containing Sm and Co as constituent elements, wherein the content of metal elements other than Sm and Co is 0.05-20 wt % with respect to the SmCo-based alloy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to SmCo-based alloy nanoparticles and to aprocess for their production.

2. Related Background Art

A large variety of magnetic materials are used in the fields of magneticrecording media and magnets. SmCo-based alloy magnetic materials havevery high coercive force and uniaxial magnetocrystalline anisotropy, andare known to exhibit high magnetic properties even as nanoparticles.SmCo-based alloys, therefore, are promising materials for use as varioustypes of magnetic materials, including magnetic recording media forhigh-density recording.

Methods for synthesis of SmCo-based alloy particles are known whichallow synthesis of SmCo₅ alloy particles by the physical method ofsputtering (for example, see Reference 1 listed below). Another methodthat has been proposed is vapor phase deposition with a cluster gun toproduce SmCo-based alloy particles (for example, see Reference 2 listedbelow).

(Reference 1) D. Weller, et al, IEEE Trans. Magn, 36, p. 10-15 (2000)(Reference 2) S. Stoyanov et al., “High anisotropy Sm—Co nanoparticles:Preparation by cluster gun technique and their magnetic properties”,JOURNAL OF APPLIED PHYSICS, Vol 93, Number 10 (2003), p. 7592

On the other hand, methods for synthesizing SmCo-based alloy particlesby chemical means have also been proposed, such as synthesis by polyolreduction (see Japanese Unexamined Patent Publication No. 2006-245313,for example) and synthesis by microwave polyol reduction (see JapaneseUnexamined Patent Publication No. 2007-128991, for example).

SUMMARY OF THE INVENTION

However, research by the present inventors has revealed that SmCo-basedparticles obtained by physical synthesis as in References 1 and 2 do nothave very high magnetic properties. Then, such physical synthesisrequire heat treatment and have low SmCo-based particle yields, they areunsuitable for industrial mass production.

On the other hand, the SmCo-based alloy particles disclosed in JapanesePatent Laid-Open No. 2006-245313 have a coercive force (Hc) of as low as500 Oe at ordinary temperature. Also, the SmCo-based alloy nanoparticlesdisclosed in Japanese Patent Laid-Open No. 2007-128991 have only beenobserved to exhibit magnetic properties at cryogenic temperature. Thelow magnetic properties of the SmCo-based alloy particles disclosed inthe aforementioned patent documents are attributed to the fact that theSmCo-based alloy particles that are produced contain large amounts ofunreacted samarium salts, since samarium salts are not easily reducedsubstances.

SmCo-based particles have therefore been desired which exhibitsatisfactorily excellent magnetic properties and have sufficiently smallparticle sizes. A process for production allowing such SmCo-based alloynanoparticles to be easily produced in mass has also been desired.

It is an object of the present invention to provide SmCo-based alloynanoparticles with sufficiently small particle sizes and satisfactorilyexcellent magnetic properties, as well as a production process thatallows the SmCo-based alloy nanoparticles to be mass-produced at highyield.

In order to achieve this object, the invention provides SmCo-based alloynanoparticles composed mainly of a SmCo-based alloy containing Sm and Coas constituent elements, wherein the content of metal elements otherthan Sm and Co is 0.05-20 wt % with respect to the SmCo-based alloy.

Such SmCo-based alloy nanoparticles can be suitably used as magneticmaterial because of their sufficiently small particle sizes andsatisfactorily excellent magnetic properties. The reason for thesatisfactorily excellent magnetic properties of the SmCo-based alloynanoparticles of the invention is that they have sufficiently lowunreacted components and comprise a SmCo-based alloy as the majorcomponent. By varying the content of the metal elements other than Smand Co, it is possible to control the magnetic properties and particlesizes of the SmCo-based alloy nanoparticles of the invention, and toimprove the degree of design freedom for magnets or magnetic recordingmedia.

The SmCo-based alloy nanoparticles of the invention preferably containthe aforementioned metal elements at 0.05-10 wt % with respect to theSmCo-based alloy. SmCo-based alloy nanoparticles with this range exhibiteven more excellent magnetic properties.

The metal elements in the SmCo-based alloy nanoparticles of theinvention preferably include at least one element selected from thegroup consisting of Au, Ag, Pt, Pd, Rh, Ru, Ir, Os, Cu, Ni, Cr, Al andMn. SmCo-based alloy nanoparticles comprising these metal elements asthe metal elements exhibit even more superior magnetic properties.

The SmCo-based alloy nanoparticles of the invention preferably haveparticle sizes of 1-30 nm. SmCo-based alloy nanoparticles in this rangecan be suitably used as magnetic recording media for high-densityrecording, for example.

The SmCo-based alloy nanoparticles of the invention are preferablyobtained by liquid synthesis wherein a strong reducing agent is added toan organic solvent containing a samarium salt, cobalt salt and metalelement salts, to reduce the samarium salt, cobalt salt and metalelement salts. Such SmCo-based alloy nanoparticles are particularlyuseful in industry as starting materials for magnetic materials, becauseof their excellent mass productivity and relatively high particle sizedistribution.

The SmCo-based alloy nanoparticles of the invention have a core-shellstructure comprising a core section and a shell section covering thecore section, and preferably the proportion of the metal elements withrespect to the SmCo-based alloy in the core section is greater than theproportion in the shell section.

According to the invention there is further provided a process forproduction of SmCo-based alloy nanoparticles composed mainly of aSmCo-based alloy containing Sm and Co as constituent elements, theprocess comprising a mixing step in which a starting material containinga samarium salt, cobalt salt and salts of metal elements other than Smand Co, as well as a protective agent, is mixed with a reducing organicsolvent, and a reduction step in which a strong reducing agent is addedto the mixture and heated to reduce the samarium salt, cobalt salt andmetal element salts.

By the process for production of SmCo-based alloy nanoparticlesdescribed above it is possible to accomplish high-yield production ofSmCo-based alloy nanoparticles with sufficiently small particle sizesand satisfactorily excellent magnetic properties. The reason for thiseffect is conjectured by the present inventors to be as follows.Specifically, it is believed that reduction of samarium salts or cobaltsalts results in rapid reduction of the salts of the metal elementsother than Sm and Co in the starting material, while the metal elementsexhibit a catalytic effect whereby they serve as nuclei for crystaldeposition which promotes deposition of the SmCo-based alloy crystals.This allows smooth synthesis of SmCo-based alloy nanoparticles with asufficiently reduced content of unreacted substances. The catalyticeffect of the metal elements adequately shortens the reaction time andinhibits grain growth of the produced SmCo-based alloy nanoparticles. Itis therefore possible to synthesize SmCo-based alloy nanoparticles thathave satisfactorily excellent magnetic properties and sufficiently smallparticle sizes. Furthermore, since the production process of theinvention employs the chemical process of reduction of the startingmaterial to synthesize the SmCo-based alloy nanoparticles, it allowsmass production of SmCo-based alloy nanoparticles at a higher yield thanphysical methods such as sputtering.

The production process of the invention preferably includes adehydration step in which the mixture obtained from the mixing step isstirred and heated for dehydration and then cooled, prior to thereduction step. This sufficiently removes moisture before the reductionstep, thus inhibiting oxidation of Sm and Co and allowing the reductionreaction of the samarium salt and cobalt salt to proceed even moresmoothly. In addition, if the mixture is cooled to near room temperatureafter dehydration and then a strong reducing agent is added prior tofurther strength and heating, it is possible to prevent the bumping thatoccurs when reduction reaction proceeds at once, and to thus reduceimpurities. That is, by adding a strong reducing agent after cooling,SmCo-based alloy nanoparticles with excellent magnetic properties can beproduced at high yield, having a further reduced unreacted substancecontent and even lower content of impurities other than the SmCo-basedalloy.

According to the production process of the invention, the metal elementspreferably include at least one element selected from the groupconsisting of Au, Ag, Pt, Pd, Rh, Ru, Ir, Os, Cu, Ni, Cr, Al and Mn.Since these metal elements are more easily reduced than samarium saltsor cobalt salts, they become rapidly reduced as crystal-generatingnuclei, thus exhibiting a more superior catalytic effect. Reduction ofthe samarium and cobalt salts can therefore be further promoted.

The strong reducing agent used in the production process of theinvention preferably contains at least one compound selected from thegroup consisting of LiAlH₄, NaBH₄, N₂H, B₂H₆ and LiBH(C₂H₅)₃. This willallow SmCo-based alloy nanoparticles to be obtained with a furtherreduced amount of residual unreacted samarium salt or cobalt salt.

According to the invention it is possible to provide SmCo-based alloynanoparticles with sufficiently small particle sizes and satisfactorilyexcellent magnetic properties, as well as a production process thatallows the SmCo-based alloy nanoparticles to be mass-produced at highyield.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a high-resolution TEM photograph and an electrondiffraction image of SmCo-based alloy nanoparticles composed mainly ofSmCo₅, according to an embodiment of the invention.

FIG. 2 is a field-emission transmission electron microscope (FE-TEM)photograph showing an example of the microstructure of a SmCo-basedalloy nanoparticle according to the invention.

FIG. 3 is a scanning transmission electron microscope (STEM) photographof the SmCo-based alloy nanoparticle shown in FIG. 2.

FIG. 4 is an XRD chart showing the results of XRD analysis ofsynthesized particles.

FIG. 5 is a graph showing the magnetic properties of the SmCo₅nanoparticles of Example 2 described hereunder.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the invention will now be explained withreference to the accompanying drawings where necessary.

The SmCo-based alloy nanoparticles of this embodiment have a meanparticle size of 1-30 nm. Thus, since the SmCo-based alloy nanoparticleswith nanosize particle sizes have excellent magnetic properties andsufficiently small particle sizes, they can be used as magnetic powderfor magnetic recording media, to increase the recording density of themagnetic recording media. The particle sizes of the SmCo-based alloynanoparticles can be measured by observation with a transmissionelectron microscope (TEM).

The preferred composition for the SmCo-based alloy as the majorcomponent of the SmCo-based alloy nanoparticles is SmCo₅. FIG. 1( a) isa high-resolution TEM photograph of the SmCo-based alloy nanoparticlescomposed mainly of SmCo₅ according to this embodiment, and FIG. 1( b) isan electron diffraction image of the SmCo-based alloy nanoparticlescomposed mainly of SmCo₅ according to this embodiment. The SmCo-basedalloy nanoparticles having this composition exhibit excellent coerciveforce (Hc) and magnetization. The composition of the SmCo-based alloynanoparticles may be confirmed by ICP optical emission spectroscopicanalysis. Because SmCo₅ has a CaCu₅-type crystal structure, it can alsobe identified by X-ray diffraction (XRD).

The SmCo-based alloy nanoparticles of this embodiment are composedmainly of a SmCo-based alloy, and contain a metal element other than Smand Co (hereinafter referred to as “third metal element”) at 0.05-20 wt% with respect to the total SmCo-based alloy. The third metal elementmay be present in the SmCo-based alloy nanoparticles as a simple metalor a metal compound.

The third metal element content is preferably 0.05-20 wt % and morepreferably 0.05-10 wt % with respect to the total SmCo-based alloy. Thethird metal element content can be confirmed by ICP optical emissionspectroscopic analysis or the like.

If the third metal element content is less than 0.05 wt % with respectto the obtained SmCo-based alloy nanoparticles during production of theSmCo-based alloy nanoparticles, the improving effect of the third metalelement will not be adequately obtained and the SmCo-based alloynanoparticle yield will be reduced, while the unreacted substancecontent will be increased, thus resulting in inferior magneticproperties. If the third metal element content is greater than 20 wt %,on the other hand, the lower proportion of SmCo-based alloy contributingto the magnetic properties will result in reduced magnetic properties.

The third metal element may be a precious metal element such as Au, Ag,Pt, Pd, Rh, Ru, Ir or Os, or a transition metal element such as Cu, Ni,Cr or Mn. Al may be mentioned as an example of a third metal elementother than these.

The SmCo-based alloy nanoparticles of this embodiment may have acore-shell structure comprising a core section with the third metalelement as the major component, and a shell section containing aSmCo-based alloy as the major component, covering the periphery of thecore section.

FIG. 2 is a field-emission transmission electron microscope (FE-TEM:JEM-2010F, trade name of JEOL Corp.) photograph showing an example ofthe microstructure of a SmCo-based alloy nanoparticle according to theinvention. As shown in FIG. 2, regions with different contrasts arepresent in a single SmCo-based alloy nanoparticle. The SmCo-based alloynanoparticles are polycrystals, and the interference pattern differsbetween the core section (center section) and the shell section (outershell section). Thus, the SmCo-based alloy nanoparticle shown in FIG. 2has a core-shell structure comprising a core section and a shell sectionhaving different compositions.

FIG. 3 is a scanning transmission electron microscope (STEM) photographof the SmCo-based alloy nanoparticle shown in FIG. 2. Elemental analysiscan be carried out using an energy dispersive X-ray spectrometer (EDS:NORAN-UTW, trade name of NORAN). As shown in FIG. 3, the SmCo-basedalloy nanoparticles of this embodiment contain the third metal element(Au) as the major component in the core section of each SmCo-based alloynanoparticle (region 1 in FIG. 3), and the SmCo-based alloy as the majorcomponent in the shell section (region 2 in FIG. 3).

A process for production of the SmCo-based alloy nanoparticles of thisembodiment will now be explained. The process for production ofSmCo-based alloy nanoparticles of this embodiment comprises apreparation step in which the samarium salt, cobalt salt and third metalelement salt are each dissolved in a reducing organic solvent to preparesolutions, a mixing step in which the samarium salt solution, the cobaltsalt solution and the third metal element salt solution are mixed toobtain a mixture, an addition step in which a protective agent is addedto the mixture obtained in the mixing step, a dehydration step in whichthe mixture containing the protective agent is heated for dehydration,and a reduction step in which a strong reducing agent is added to thedehydrated mixture to reduce the samarium salt, cobalt salt and thirdmetal element salt, in order to obtain SmCo-based alloy nanoparticlescontaining the third metal element. Each of these steps will now beexplained in detail.

(Preparation Step)

The Sm (samarium) salt used for this embodiment may be samariumacetylacetonate hydrate ([CH₃COCH═C(O—)CH₃]₃Sm.xH₂O), and the Co(cobalt) salt may be cobalt acetylacetonate ([CH₃COCH═C(O—)CH₃]₃Co).These salts are readily soluble in organic solvents and relativelyeasily reduced, and are therefore preferably used from the viewpoint ofobtaining SmCo-based alloy nanoparticles with small particle sizes andhigh purity.

The third metal element salt may be an acetylacetonate salt.Acetylacetonate salts are preferred for use because they are readilysoluble in organic solvents.

The organic solvent used to dissolve the samarium salt, cobalt salt andthird metal element salt is preferably one with a high boiling point. Aspecific example is 1,2-hexadecanediol, which has reducing action.

Dissolution of the samarium salt, cobalt salt and third metal elementsalt in 1,2-hexadecanediol in the preparation step produces a solutioncontaining the samarium salt, a solution containing the cobalt salt anda solution containing the third metal element salt. In order to preventoxidation of the Sm, Co and third metal element, the preparation step ispreferably carried out in an inert gas atmosphere such as nitrogen orargon.

(Mixing Step)

In the mixing step, the solutions prepared in the manner described aboveare mixed. There are no particular restrictions on the order of mixing,and the three solutions may be combined simultaneously or two of thesolutions may be combined and the remaining one mixed with the obtainedmixture.

The amount of third metal element salt solution used is preferably0.01-0.5 mol in terms of the third metal element with respect to 1 molof the total of the samarium and cobalt elements in the samarium andcobalt salts.

The mixing step is preferably carried out in an inert gas atmospheresuch as nitrogen or argon in order to prevent oxidation of the Sm, Coand third metal element.

(Addition Step)

In this step, a protective agent is added to the mixture obtained in themixing step. The protective agent has the function of protecting theSmCo-based alloy nanoparticles that are produced, and oleic acid,oleylamine, polyvinylpyrrolidone or polyvinyl alcohol is preferablyused. The protective agent may also be used in the form of a solution inan organic solvent such as ether.

The amount of protective agent added is preferably 0.1-20 mol withrespect to 1 mol of the total of the samarium and cobalt elements in thesamarium and cobalt salts.

(Dehydration Step)

The dehydration step is a step in which the mixture is stirred andheated under an inert gas flow or under reduced pressure after inert gassubstitution, for dehydration. Sufficiently reducing the moisture in themixture can satisfactorily inhibit oxidation of the reduced Sm or Co. Itwill thus be possible to obtain SmCo-based alloy nanoparticles at aneven higher yield.

Heating of the mixture is preferably carried out at 110-220° C. for 1-24hours. The mixture is then preferably cooled to room temperature, andmore preferably cooled to 10-30° C. This will allow reduction with thestrong reducing agent in the reduction step described hereunder to beaccomplished in a more reliable manner, so that high-purity SmCo-basedalloy nanoparticles are obtained. If the addition of a strong reducingagent is under high-temperature conditions (for example, 50° C. andhigher) without cooling, the reduction with the strong reducing agentwill occur all at once, thus tending to result in production ofimpurities, such as simple cobalt (Co metal), in addition to theSmCo-based alloy, and lowering the SmCo-based alloy content.

(Reduction Step)

The reduction step is a step in which, after the strong reducing agenthas been added to the mixture following the dehydration step, themixture is stirred and heated to thoroughly reduce the samarium salt,cobalt salt and third metal element salt and produce SmCo-based alloynanoparticles containing the third metal element.

The strong reducing agent used may be at least one compound selectedfrom the group consisting of LiAlH₄, NaBH₁, N₂H₄, B₂H₆ and LiBH(C₂H₅)₃.These strong reducing agents are preferably added to the mixture afterdissolution in an organic solvent such as an alcohol.

After addition of the strong reducing agent, an oil bath or mantleheater is used to keep the mixture at 150-320° C. and preferably250-280° C. for 1-3 hours for heated reflux, to accomplish reductionwith the strong reducing agent and the reducing organic solvent andobtain a reaction mixture. The reduction reaction reduces the samariumsalt, cobalt salt and third metal element salt. Upon removal of thesolvent from the obtained reaction mixture, SmCo-based alloynanoparticles containing the third metal element are obtained.

Since the third metal element salt is more easily reduced than thesamarium salt and cobalt salt in the reduction reaction described above,it tends to be reduced earlier than the samarium salt and cobalt salt.Therefore, the third metal element presumably acts as nucleus originsfor deposition of the SmCo-based alloy. The presence of the third metalelement ensures even smoother reduction of the samarium salt and cobaltsalt, so that the obtained SmCo-based alloy nanoparticles containing thethird metal element have a sufficiently reduced impurity content.

The SmCo-based alloy nanoparticles containing the third metal elementcan be suitably used as magnetic material because of both theiradequately small particle sizes and excellent magnetic properties. Theparticle sizes of the SmCo-based alloy nanoparticles may be adjusted byvarying the amount of protective agent added and the third metal elementcontent within the ranges according to the invention, or by varying thetemperature and time for heated reflux in the reduction step.

The production process of this embodiment allows high-yield synthesis ofSmCo-based alloy nanoparticles containing the third metal element. Thenanoparticles obtained by the production process of this embodiment mayalso include nanoparticles composed mainly of a SmCo-based alloy withoutthe third metal element, or nanoparticles of the third metal element.

The embodiment described above is only a preferred embodiment of theinvention, and the invention is in no way limited thereto.

EXAMPLES

The present invention will now be explained in greater detail based onexamples and comparative examples, with the understanding that theseexamples are in no way limitative on the invention.

Examples 1-8 Synthesis of SmCo-Based Alloy Nanoparticles

A first solution was prepared by dissolving 0.33 mmol of samariumacetylacetonate hydrate ([CH₃COCH═C(O—)CH₃]₃Sm.xH₂O) and 0.02-0.5 mmolof the third metal salts listed in Table 1 in 2 ml of 1,2-hexadecanediol(CH₃(CH₂)₁₃CH(OH)CH₂OH) under a nitrogen atmosphere.

Separately from this solution, there was prepared a second solution bydissolving 1.67 mmol of cobalt acetylacetonate ([CH₃COCH═C(O—)CH₃]₃Co)in 2 ml of 1,2-hexadecanediol under a nitrogen atmosphere.

Also, a third solution was prepared by dissolving 3.0 mmol of oleic acid(CH₃ (CH₂)₇CH═CH(CH₂)₇COOH) and 3.0 mmol of oleylamine(CH₃(CH₂)₇CH═CH(CH₂)₈NH₂) in 40 ml of octyl ether ([CH₃(CH₂)₇]₂O) underan inert gas atmosphere (for example, nitrogen or argon).

The first solution, the second solution and the third solution were thenmixed for about 12 hours using a mechanical stirrer under a nitrogenatmosphere to obtain a mixture.

In order to remove the water in the mixture, a three-necked flaskcontaining the mixture was heated in an oil bath at 200° C. under anitrogen stream and these conditions were maintained for about 1 hour,after which it was cooled to room temperature (about 20° C.). To thecooled mixture there was added absolute ethanol dissolving 6 mmol ofsodium borohydride (NaBH₄), as a strong reducing agent.

The temperature of the oil bath was then raised and the reaction mixturewas kept at 250-280° C. for 1-3 hours of heated reflux. After cooling toroom temperature, an ultrafilter was used for filtration and dehydratedethanol or the like was used for solution exchange and washing of theparticles. Next, an evaporator was used to remove the solvent attachedto the particles, and vacuum drying was carried out for 10 hours at 40°C. to obtain particles for Examples 1-8.

[Evaluation of Particles]

The particles obtained in Examples 1-8 were observed (3,000,000magnification) with a high-resolution TEM (JEM-3010, trade name of JEOLCorp.). Also, 100 particles were randomly selected from the electronmicroscope image, and the mean particle size was calculated. The meanparticle sizes of the synthesized particles are listed in Table 1.

The obtained particles were also examined by X-ray diffraction (XRD) andICP optical emission spectroscopic analysis. FIG. 4 shows the XRDresults for the synthesized particles, wherein the upper chart is an XRDchart for the particles obtained in Example 2. These XRD measurementresults confirmed that the particles synthesized in Example 2 areparticles composed mainly of SmCo₅ (SmCo₅ particles) with an adequatelyreduced content of impurities such as oxides. The results of electrondiffraction image analysis using the high-resolution TEM also confirmedthat the synthesized particles were SmCo₅.

The particles of Example 1 and Examples 3-8 were also confirmed by XRDand high-resolution TEM analysis to be SmCo₅ particles with adequatelyreduced contents of impurities such as oxides. The contents of the thirdmetal elements in the particles of Examples 1-8 as determined by ICPoptical emission spectroscopic analysis are also listed in Table 1.

The yields of SmCo₅ particles in Examples 1-8 are also shown in Table 1.The yields are the total masses of Sm and Co elements as measured by ICPoptical emission spectroscopy, with respect to the theoretical producedmass of SmCo₅ as calculated from the amounts of Sm and Co used.

The evaluation results described above confirmed that the SmCo₅nanoparticles containing third metal elements were obtained at highyield.

[Evaluation of Magnetic Properties]

The coercive forces (Hc) of the SmCo₅ nanoparticles of Examples 1-8 weremeasured using a VSM (Vibrating Sample Magnetometer) (VSM-5, trade nameof Toei Industry Co., Ltd.), under conditions with an applied magneticfield of 20 kOe at 25° C. The results of coercive force measurement areshown in Table 1. FIG. 5 is a graph showing the magnetic properties ofthe SmCo₅ nanoparticles of Example 2. The SmCo₅ nanoparticles of Example1 and Examples 3-8 exhibited satisfactorily excellent magneticproperties, similar to the SmCo₅ nanoparticles of Example 2.

Example 9

Particles were obtained by synthesis in the same manner as Example 2,except that lithium aluminum hydride (LiAlH₄) was used as the strongreducing agent instead of sodium borohydride. Evaluation of theparticles and their magnetic properties in the same manner as Example 2revealed a high yield of SmCo₅ nanoparticles with excellent magneticproperties, containing Cu as the third metal element. The particle size,third metal element content, yield and coercive force (Hc) of the SmCo₅nanoparticles were measured and the results are shown in Table 1.

Comparative Example 1

SmCo₅ nanoparticles were synthesized in the same manner as Example 1,except that the third metal salt (palladium acetylacetonate) and sodiumborohydride were not used. The particles and their magnetic propertieswere evaluated in the same manner as Example 1. The lower chart of FIG.4 is an XRD chart for the particles obtained in Comparative Example 1.The analysis results confirmed that the particles contained an abundantamount of compounds other than the SmCo-based alloy (such as Sm oxidesand Co oxides), and therefore were not composed mainly of the SmCo-basedalloy. The particle size, yield and coercive force (Hc) of the obtainedparticles are shown in Table 1.

Comparative Example 2

Particles were obtained by synthesis in the same manner as Example 1,except that no sodium borohydride was used. The particles and theirmagnetic properties were evaluated in the same manner as Example 1. Themiddle chart of FIG. 4 represents XRD analysis for the particlesobtained in Comparative Example 2. The analysis results confirmed thatthe particles contained compounds other than the SmCo-based alloy (suchas Sm oxides and Co oxides), and therefore were not composed mainly ofthe SmCo-based alloy. The particle size, third metal element content inthe particles, yield and coercive force (Hc) of the obtained particlesare shown in Table 1.

Comparative Example 3

Particles were obtained by synthesis in the same manner as Example 1,except that the third metal element salt (palladium acetylacetonate) wasnot used. The particle size, yield and coercive force (Hc) of theparticles are shown in Table 1. The coercive force (Hc) of the particlesof Comparative Example 3 was lower than that of the SmCo₅ nanoparticlescontaining third metal elements of Examples 1-9.

TABLE 1 Strong Mean particle Third metal element reducing size YieldContent Hc Third metal element salt agent (nm) (wt %) Type (wt %) (Oe)Example 1 [CH₃COCH═C(O—)CH₃]₂Pd NaBH₄ 6 45 Pd 0.76 1010 Example 2[CH₃COCH═C(O—)CH₃]₂Cu NaBH₄ 5 66 Cu 0.79 1290 Example 3[CH₃COCH═C(O—)CH₃]₂Cu NaBH₄ 12 43 Cu 0.07 1000 Example 4[CH₃COCH═C(O—)CH₃]₂Cu NaBH₄ 3 52 Cu 19.46 1030 Example 5[CH₃COCH═C(O—)CH₃]₂Ni•2H₂O NaBH₄ 5 63 Ni 0.74 1240 Example 6[CH₃COCH═C(O—)CH₃]₂Pt NNaBH₄ 4 55 Pt 0.78 1430 Example 7 AuCl NaBH₄ 5 58Au 0.80 1280 Example 8 [CH₃COCH═C(O—)CH₃]₃Cr NaBH₄ 7 41 Cr 0.69 1040Example 9 [CH₃COCH═C(O—)CH₃]₂Cu LiAlH₄ 6 49 Cu 0.75 1120 Comp. Example 1None None 30 25 None 0.00 150 Comp. Example 2 [CH₃COCH═C(O—)CH₃]₂Pd None9 39 Pd 0.78 700 Comp. Example 3 None NaBH₄ 15 30 None 0.00 650

1. SmCo-based alloy nanoparticles composed mainly of a SmCo-based alloycontaining Sm and Co as constituent elements, wherein the content ofmetal elements other than Sm and Co is 0.05-20 wt % with respect to theSmCo-based alloy.
 2. SmCo-based alloy nanoparticles according to claim1, wherein the content of the metal elements is 0.05-10 wt % withrespect to the SmCo-based alloy.
 3. SmCo-based alloy nanoparticlesaccording to claim 1, wherein the metal elements include at least oneelement selected from the group consisting of Au, Ag, Pt, Pd, Rh, Ru,Ir, Os, Cu, Ni, Cr, Al and Mn.
 4. SmCo-based alloy nanoparticlesaccording to claim 1, wherein the particle sizes are 1-30 nm. 5.SmCo-based alloy nanoparticles according to claim 1, which are obtainedby liquid synthesis wherein a strong reducing agent is added to anorganic solvent containing a samarium salt, a cobalt salt and metalelement salts, to reduce the samarium salt, the cobalt salt and themetal element salts.
 6. SmCo-based alloy nanoparticles according toclaim 1, having a core-shell structure comprising a core section and ashell section covering the core section, wherein the proportion of themetal elements with respect to the SmCo-based alloy in the core sectionis greater than the proportion in the shell section.
 7. A process forproduction of SmCo-based alloy nanoparticles composed mainly of aSmCo-based alloy containing Sm and Co as constituent elements, theprocess comprising: a mixing step in which a starting materialcontaining a samarium salt, a cobalt salt and salts of metal elementsother than Sm and Co, as well as a protective agent, is mixed with areducing organic solvent; and a reduction step in which a strongreducing agent is added to the mixture and heated to reduce the samariumsalt, the cobalt salt and the salts of the metal elements.
 8. A processfor production of SmCo-based alloy nanoparticles according to claim 7,further comprising a dehydration step in which the mixture obtained fromthe mixing step is stirred and heated for uniform dissolution anddehydration and then cooled, prior to the reduction step.
 9. A processfor production of SmCo-based alloy nanoparticles according to claim 7,wherein the metal elements include at least one element selected fromthe group consisting of Au, Ag, Pt. Pd, Rh, Ru, Ir, Os, Cu, Ni, Cr, Aland Mn.
 10. A process for production of SmCo-based alloy nanoparticlesaccording to claim 7, wherein the strong reducing agent contains atleast one compound selected from the group consisting of LiAlH₄, NaBH₁,N₂H₄, B₂H₆ and LiBH(C₂H₅)₃.